A heat assisted magnetic recording (HAMR) head for high density data storage generally requires laser thermal assistance to locally heat a media. FIG. 1 shows a schematic diagram of a conventional optical delivery system 100. The conventional optical delivery system 100 uses a waveguide 102 which is integrated in the HAMR head 104 and close to a recording pole 106. The waveguide 102 guides the light from a light source (e.g. a laser diode) (not shown) to a media 108. The media 108 is locally heated before reaching the recording head 104.
FIG. 2a shows a schematic diagram of the waveguide 102. The waveguide 102 integrated in the write head 104 (FIG. 1) has a first cladding layer 202, a second cladding layer 204 and a core layer 206 disposed between the first cladding layer 202 and the second cladding layer 204. For example, the core layer 206 of the waveguide 102 includes silicon nitride and has a thickness of about 300 nm. The refractive index of the core layer 206 is 2.0 and the refractive index of the first cladding layer 202 and the second cladding layer 206 is 1.5 at a wavelength of about 780 nm. The waveguide 102 usually has a much smaller spot size at least in one direction (for example, in a direction along a y-axis) when compared to a light beam 208 from the light source (not shown).
FIG. 2b shows a graph 210 of amplitude plotted against distance. Plot 212 shows an Eigenmode profile of the waveguide 102 and plot 214 shows a field profile of the laser beam 208. From plot 212, it can be observed that the mode field diameter of the waveguide 102 is about 0.4 μm. From plot 214, it can be observed that the input laser beam 208 has a Gaussian profile
      E    ⁡          (      y      )        =      exp    (                  -        4            ⁢                          ⁢                        y          2                          W          e          2                      )  in a y-z plane, whereby y is a distance extending in a direction along a y-axis and We is a spot size diameter of the laser beam 208 (e.g. the spot size diameter can be We=4 μm). Plot 212 and plot 214 show a mismatch of the spot size between the laser beam 208 and the eigenmode of the waveguide 102. As such, an optical device is required to couple the light beam 208 from a light source (not shown) into the waveguide 102.
FIG. 3 shows a schematic diagram of a conventional recording head 302. The recording head 302 includes a waveguide 304 to guide the light 305 from a light source (e.g. laser diode) (not shown) to a media 306. The waveguide 304 uses a grating structure 308 to couple the light from the light source (not shown) into the waveguide 304 (as described for example in US 2004/0001420 A1 “Heat assisted magnetic recording head with a planar waveguide”). However, the grating based coupler 308 has a limited coupling efficiency and has a strict requirement on the assembling of the light source (not shown) as compared to the butt-coupling.